1. Field of the Invention
The present invention relates to an electronic camera, or more particularly, to an electronic camera for automatically controlling exposure by utilizing an output of a solid-state imaging device.
2. Description of the Related Art
In the past, a digital camera capable of automatically controlling exposure by utilizing an output of a solid-state imaging device such as a CCD, in other words, capable of carrying out AE imaging has been known.
For example, a TV camera composed of a solid-state imaging device, a diaphragm, and a video signal amplifier has been disclosed in Japanese Unexamined Patent Publication No. 5-236338. The solid-state imaging device has a condenser micro-lens located near a light receptor. The diaphragm controls an amount of light incident on the solid-state imaging device. To the video signal amplifier, a video signal is supplied from the solid-state imaging device. When an output of the solid-state imaging device exhibits an average voltage level, the TV camera guarantees a minimum illuminance for an object irrespective of whether the diaphragm has been changed in diameter.
This TV camera detects an f-number, i,e., information about the zoom position and corrects the gain.
On the other hand, a technological means for correcting a change in sensitivity of a solid-state imaging device by adjusting the setting of an f-number for a camera lens has been disclosed in Japanese Unexamined Patent Publication No. 6-178198.
The technological means stores sensitivity levels associated with f-numbers as data items in relation to a plurality of solid-state imaging devices, and corrects the gain according to the diameter of the aperture.
However, in this type of digital camera which performs AE imaging, as the diaphragm for the micro-lens attached to the CCD is opened more widely, the sensitivity of the CCD tends to deteriorate. When photography (recording) is started in response to a shutter trigger, if the diaphragm is changed in diameter from a previous setting, an error proportional to the change in sensitivity of the CCD occurs in exposure precision. If the gain is adjusted in order to correct the exposure precision as proposed by the prior art, there arises a problem in that the noise of an image to be recorded becomes intensified.
FIG. 5 is a graph indicating voltage levels of outputs of a CCD or a solid-state imaging device associated with different f-number settings for AE imaging carried out by a conventional digital camera employing a solid-state imaging device. Table 1 demonstrates the relationship between an f-number setting for AE imaging and an f-number setting for real exposure.
As shown in FIG. 5, assume that when the sensitivity of a CCD does not deteriorate as the diaphragm is narrowed; AE imaging is carried out with a designated f-number FAE. A shutter speed TAE is found to provide a proper exposure level (brightness). Thereafter, real exposure (recording) is carried out with the f-number changed to FREC in response to a shutter trigger. In this case, a shutter speed TREC providing the proper exposure level is expressed as follows:
On the other hand, when the CCD sensitivity decreases upon diaphragm narrowing, if real exposure (recording) were to be carried out by designating a shutter speed calculated according to the above expression, the exposure level would be deviated from the proper value.
The first object of the present invention is to provide an electronic camera capable of carrying out a proper exposure or recording irrespective of the sensitivity of a solid-state imaging device.
The second object of the present invention is to provide an electronic camera capable of, although the camera may have zoom capacity, carrying out exposure or recording with a high level of precision irrespective of the sensitivity of a CCD.
Briefly, an electronic camera of the present invention comprises a CCD, a diaphragm, an EEPROM, and a CPU. The CCD has a condenser micro-lens opposed to a light receptor. The diaphragm is variable in relation to a plurality of f-numbers. In the EEPROM, data items of outputs of the CCD associated with the plurality of f-numbers or ratios of the outputs thereof are stored in advance. The CPU calculates an exposure time according to an output of the CCD associated with a given f-number, control the exposure, and corrects the exposure time according to data stored in the EEPROM during recording.